Lets say I'm mining bitcoins, and I've just received a new block that someone else has mined. What encourages me to send that block to anyone else.
I can understand how the original miner wants to send the block around the network (so their block becomes part of the chain) but what incentive do other nodes have to send it around? Wouldn't it be better to not to spread the block, but then build your own block on top of it, in which case you'll then be two blocks ahead and more likely to be added to the chain?
Of course, if everyone does this the result would be terrible, but what incentive is there to encourage people (other than the original miners) to spread blocks besides altruism?